To read the original document by R.V. Pound and G.A. Rebka select this link: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.4.337

• The text in italics is copied from that url
• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Introduction

The article starts with the following sentence.

### 1. Overview

However, if the two systems are in a gravitational field then the photon may undergo gravitational redshift as it travels from the first system to the second, causing the photon frequency observed by the receiver to be different to the frequency observed by the emitter when it was originally emitted.
This sentence is misleading. The issue that when a photon travels in a gravitational field its energy, in casu its frequency , can change. There are two possibilities, based on the law E=hf :
1. Its energy, its frequency can increase, which is the same as a decrease in wave length. This is called a blue shift.
2. Its energy, its frequency can decrease, which is the same as a increase in wave length. This is called a red shift.
The difference depents if the photon travels towards a mass or away from a mass.
Another possible source of redshift is the Doppler effect: if the two systems are not stationary relative to one another then the photon frequency will be modified by the relative speed between them.
Studying this in detail there are also different situations:
1. The source is stationary and destination moves away. In that case there is a increase in wave length i.e. redshift.
2. The source is stationary and destination moves towards. In that case there is a decrease in wave length i.e. blueshift.
3. The source is moves away and destination stationary. In that case there is a increase in wave length i.e. redshift.
4. The source is moves towards and destination stationary. In that case there is a decrease in wave length i.e. blueshift.
The question to answer is if the two situations 1 and 3, when single photons are involved, are identical.
To counteract the effect of gravitational blueshift, the emitter was moved upwards (away from the receiver) causing the photon frequency to be redshifted, according to the Doppler shift formula: etc.
The emitter is at the top. The receiver is down.
Accordingly to above:
When the source moves away and destination stationary then there is a increase in wave length i.e. redshift.
This increase is cancelled when the photon travels towards a mass i.e falls downwards towards the earth.

### Reflection 1 - The Pound-Rebka experiment

The Pound-Rebka experiment involves two issues:
1. What is the physical explanation of the experiment.
2. How to quantify the experiment.
The experiment involves a tower. At the top there is a source which emits a photon. This photon moves towards the bottom where the photon is detected/measured/absorbed.
The idea behind the experiment is to use the same material to emit and to absorb a photon in order to unravel what gravity does with a photon.
The physical theory behind the experiment is that when a photon falls in a gravitational field its energy increases. To counter act this increase the source is moved away with a speed v. When that is done and the speed is 'correct' the photon will be absorbed. That means the theory is correct. However what does that mean.

When you read the article the explanation is in the frequency of the photon. However the frequency of the photon is not directly measured.

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Created: 25 June 2018

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